Luminescent material and method of manufacture



Patented Oct. 1946 LUMINESCENT MATERIAL AND 'METHOD OF MANUFACTUREHumboldt W. Leverenz, South Orange, N. .L, assignor to Radio Corporationof America, a corporation of Delaware No Drawing. Application June 11,1942, Serial No. 446,626

9 Claims.

My invention relates to luminescent materials and their preparation andparticularly to the preparation of luminescent zinc compounds.

It has been customary in the art to synthesize luminescent materials orphosphors of zinc and cadmium sulphides by various methods although themajority of these methods result in a product which has only moderatephosphorescent properties. While I have disclosed in my copendingapplication, Serial No. 412,687, filed September 27, 1941, a sulphidephosphor having relatively high phosphorescence of relatively longduration following excitation to luminescence, even greater intensitiesand longer periods of phosphorescence following excitation are desirablein certain applications in which such phosphors are utilized. Forexample, in aircraft position and distance indicating equipment, it isquite desirable to provide a cathode ray tube having a long persistencephosphor, such persistence rendering the tube suitable for portrayingthe trajectory of an approaching aircraft. Sulphide phosphors areusually characterized by their low resistance to heat. For example, thebaking to which they are usually subjected during cathode ray tubemanufacture reduces their eificiency and for certain applications theyare wholly unsuitable for use because of their low resistance to heat.

My invention has as an object, the production of zinc compoundspossessing high luminescent properties and particularly highphosphorescence without undue sacrifice of fluorescence. Another objectis to provide a material which is phosphorescent over a relatively longperiod of time following excitation to luminescence. A further object isto provide phosphors which are insensitiv to heat following synthesis sothat they can be used in applications where high temperature processing,such as encountered in tube manufacture, has little or no effect on thephosphor efficiency. These and other objects, features and advantages ofmy invention are obtained in accordance therewith by combining a zincsulphide and a fluoride of zinc to form a mixed sulphide-fluoride masswhich is co-crystallized to form mixed sulphide-fluoride crystals.Further in accordanc with my invention I provide a material in which thesulphide of zinc either may be replaced in part by zinc oxide or thezinc may be replaced in part by cadmium with or Without zinc oxide priorto the formation of the mixed sulphide-fluoride crystals. Furthermore,in accordance with my invention I control the purity of the constituentswithin very narrow limits to provide a material having high fluorescentand phosphorescent efiiciencies. I have found that such properties areexceedingly dependent upon the purification of the constituents of myphosphor. These requirements for exceedingly pure constituentsapparently are due in part to the characteristics of my preferredconstituents.

The above-mentioned objects are attained in part by controlling theconstituent content of certain impurities. For example, the initialconstituents from which I synthesize my phosphor material andparticularly the zinc salt solution from which I form a sulphide shouldhave less than the following percentage of impurities:

Per cent Mn 0.0008 F'e 0.0005 As 0.0008 Ni 0.00001 I prefer tosynthesize my phosphor material starting with zinc halide or, morepreferably, zinc sulphate and I remove a large proportion of inherentlycontained iron and certain other metals by an oxidation process to forminsoluble compounds which are precipitated and removed. Moreparticularly I make a solution of zinc sulphate ZnSO4-7H2O) in theproportion of approximately 50 grams to 33 milliliters of pure water andadd thereto a solution of hydrogen peroxide in an amount approximating 5milliliters of 30 per cent saturated solution. I then add ammoniumhydroxide until a slight White turbidity is produced. Following theaddition of ammonium hydroxide I bring the solution to a boil as rapidlyas possible to remove certain volatile products, such as ammonia gas,and continue to boil until ammonia fumes are unnoticeable. I then allowthe solution to stand for at least 24 hours followed by decanting or byfiltering to remove the precipitate. In addition to the removal of iron,these steps likewise remove aluminum, nickel and copper. Centrifugingmay be employed to reduce the time necessary to bring down theprecipitate.

Following the removal of the above impurities, the pH of the zincsulphate solution should be between 4-7, any adjustment, if necessary,being made by adding an acid, such as sulphuric acid, following which Iheat the zinc sulphate solution to approximately C. and allow it totrickle over pure zinc for the purpose of removing all elements belowzinc in the electromotive force series of the elements. The treatmentwith zinc metal may be accomplished at the very outset, such as byreacting an acid such as hydrochloric or sulphuric acid with zinc metalor zinc oxide in the presence of excess zinc metal. Following such atreatment the purity of the zinc sulphate solution will be approximatelyas indicated above, although to further assure removal of anyexceedingly small amount of manganese, lead and copper, I prefer toelectrolyze the solution between platinum gauze electrodes at' 2-2.2VOltS' for a period ranging between 50-400 hours. To still furtherassure purity I then fractionally precipitate a small portion of thezinc as a zinc sulphide by passing a small quantity of well-purifiedhydrogen sulphide into the zinc sulphate solution following which theprecipitated zinc sulphide is removed by filtration and discarded. Thisfractional precipitation step may be repeated to further lower theconcentrations of undesirable impurities.

In accordance with my invention I precipitate zinc sulphide and zincfluoride (with or without an activator, such as copper, silver or gold)from the previously prepared zinc sulphate solution or alternatively Iprecipitate only a pure zinc sulphide and then add to this sulphide afluoride in a form which will convert a portion of the zinc sulphide tozinc fluoride, or still alternatively add zinc fluoride to the preparedzinc sulphide prior to calcining or heating of the mixture to producethe sulphide-fluoride phosphor. The amount of zinc fluoride eithercoprecipitated with the zinc sulphide or added thereto as a fluoride isrelatively critical to obtain the objects of my invention. For example,the fluoride range is preferably between 0.1% and 15% by weight based onthe zinc sulphide content. I have found that in addition to the zincfluoride a limited quantity of zinc oxide in the resultant phosphor i'sbeneficial in enhancing the phosphorescent properties thereof; andfurthermore, that while a single activator, such as copper, ispreferably incorporated in my phosphor in known and controlled amounts,it is particularly with the teaching of my invention to utilize aplurality of activators selected from the group consisting of copper,silver and gold. As indicated above, my phosphor consists of a mixedcrystal composition and it should be understood that the zinc fluorideand zinc oxide which may be present are not necessarily present asstrict chemical combinations in the final product. These materialsmerely symbolize the presence of fluorine and oxygen or their ions inthe sulphide phosphor. The resultant phosphor may be symbolized asZnS(ZnF2).e(ZnO)1/:Cu, wherein the subscripts a: and y arerepresentative of the amount of fluoride and oxide in the combinedphosphor crystal composition by weight based on the zinc sulphidecontent. More particularly the fluoride and oxide content should bewithin limits such that x plus y is equal to 0.1% to 15% based on thezinc sulphide content of which the value 3 is preferably equal to orgreater than zero but not greater than 5 As one specific example ofpreparing the sulphide-fluoride mixture prior to calcination, Iprecipitate zinc sulphide from the acidified zinc sulphate solution bybubbling purified and thoroughly washed hydrogen sulphide through thesolution to form a precipitate of zinc sulphide. Simultaneously with orpreferably following the formation of the zinc sulphide precipitate, Iprecipitate zinc fluoride by adding gaseous or liquid hydrogen fluorideor an aqueous solution of hydrogen fluoride to the zinc sulphatesolution. I'

obtain somewhat greater purity by forming the zinc fluoride byprecipitation subsequent to the preferably performed in platinumreceptacles to prevent formation of undesired fluorides although thismay be obviated by forming the zinc fluoride from the filteredprecipitated Zinc sulphide or adding it thereto following precipitationand filtering. I then thoroughly mix the sulphide-fluoride mixture witha platinum rod or spatula. If an unactivated phosphor is desired, themixture is dried in an oven or over a water bath, broken up and fired ata temperature of from 750 C. to 1600 C. for aperiod of from 10 minutesto 10 hours depending upon the particle size desired and the temperatureof firing. However, if an activated material is desired, the activatormetal is added to the semi-fluid paste as a soluble salt in a watersolution and then the material is dried and fired as above. As explainedbelow in greater detail certain fluxing agents may be added to theactivated or unactivated material to assist the thermosynthesis of thephosphor.

Asa specific example of one teaching of my invention, I add to 100 gramsof the pure precipitated zinc sulphide and contained in a platinumcrucible a quantity of zinc fluoride between 0.5% and 5% based on thezinc sulphide by weight such as 2 grams of zinc fluoride. AlternativelyI add a sufficient quantity of a substance, such as hydrogen fluoride orammonium fluoride, which will react with the zinc sulphide to yield thedesired quantity of zinc fluoride. The specific quantity of zincfluoride cited above, that is, 2% based on the zinc sulphide content, ispreferred inasmuch as sulphides of this composition exhibit optimumphosphorescence under 3650 A. radiation Without undue aggregation. Oneor more activator metals selected from the group consisting of copper,silver and gold may be used and I prefer to add this activator as asoluble salt of the metal either preceeding or following theprecipitation of the zinc sulphide or following the preparation of thezinc sulphide-zinc fluoride mixture. Since the sulphides of copper,silver and gold are extremely insoluble, it is immaterial whether theirsoluble salts be added before or after the precipitation of the Zincsulphide. Thus, for example, to the purified zinc sulphate solution orto the wet mixture of zinc sulphide and zinc fluoride I add an aqueoussolution containing the activator metal or metals as soluble salts. Ihave found for the above preparations, the amount of copper may varyover wide limits; for example, the amount may vary from 0.0000l% to 0.1%molal with respect to the zinc sulphide content, although I prefer inthe above example to add 0.01 gram of copper as the fluoride. I havefound that the use of silver or gold as an activator with the copperincreases the phosphorescence of the resultant material and the amountof copper, silver and gold may vary from 0.00001% to 0.1% molal, eachwith respect to the zinc sulphide content. In addition I have found thatin a multi-activated phosphor the quantity of the individual activatorsmay decrease in the order of silver, copper and gold, the gold beingless than the copper and the copper being less than the silver. Therelative proportion of these three activators may be one part gold, twoparts copper and four. parts silver. However, for varying thephosphorescent characteristic these relative proportions may vary aboveand below the optimum proportion. I have found that the period of usefulphosphorescence in a silver-gold activated material increases inaccordance with the amount of gold with respect to silver and similarly.with a copper-silver or copper-gold activator, the period of usefulphosphorescence increases with the quantity of copper and goldrespectively. The use of 0.1% of silver and 005% copper by weight, toweight of zinc sulphide, gives optimum phosphorescence under cathode rayexcitation.

To aid in the thermo-synthesis or calcination of my phosphor I havefound that specific preferred fluxing agents or catalysts may be mixedwith the sulphide-fluoride mixture prior to the calcining or firingsteps. I specifically avoid such commonly used fluxing agents ormaterials as boric acid, magnesium-fluoride and calcium carbonate, whichdo not completely evaporate or form readily removable compounds, byusing a sodium or potassium chloride or bromide fluxing agent whichcontrols the crystal growth and phosphor formation upon calcination tofar better advantage than other fluxes in the preparation of theconventional sulphide phosphors. The sodium or potassium chlorides (orbromides) react with the zinc sulphide to form zinc chloride (orbromide) which sublimes at 732 C. leaving easily soluble sodium orpotassium sulphides. I, therefore, add to the wet zinc sulphide-zincfluoride an aqueous solution containing 2 grams of sodium or potassiumchloride (or bromide) following which I evaporate to dryness, such as inan oven or under an infra-red lamp or over a water bath. I then placethe material in the powdered state in a clean quartz crucible although aplatinum crucible may be used where the life of the crucible is aneconomic factor in commercial preparation. contents in a clean electricfurnace and raise the temperature of the furnace to between 750" C. and1600 C. and maintain this temperature for a time interval of fromminutes to 10 hours, the total heating time and temperature dependingupon the phosphorescence intensity and particle size desired in theresulting phosphor material, the particle size and phosphorescenceintensity increasing with the time of calcination. Lower temperatures,such as 750 C.-1000 C. yield smaller crystals, but higher temperatures,such as 1100 C.-1300 C. give higher phosphorescent intensity. Followingcrystallization the material is removed from the crucible, thoseportions which reacted therewith being detected under ultra-violetexcitation and usually discarded whereupon the crystallized material inaccordance with my invention is then washed with vigorous stirring inpure distilled water saturated with hydrogen sulphide. I have found thatI then place the crucible and its 1 the group of metals consisting ofzinc and cadmium wherein the fluoride is between 0.1% and by weightbased on the weight of said 7 sulphide.

wherein M is a metal selected from the group of metals consisting ofzinc and cadmium, F is fluorine, S is sulphur, O is oxygen, and :r and ythe period of Washing should be short and that preferably the materialmay be washed two to three times in pure water saturated with hydrogensulphide. The material should not be subjected to the washing water fora period in excess of 10-100 minutes (shorter time for higher fluoridecontent) following which the material is immediately dried at atemperature from. 100 C. to 250 C. whereupon it is ready for use. I havefound that my product not only has exceptionally high phosphorescentproperties and excellent stability to heat but that such phosphorescenceis not obtained ata sacrifice of fluorescence and further that my methodof production may provide a smooth flowing, substantially nonaggregated,non-flocculent material which may be easily applied during thesubsequent manufacture of luminescent coatings.

I claim:

1. A luminescent phosphor essentially consisting of a thermallycrystallized combination of a sulphide and fluoride of a metal selectedfrom represent the weight of fluoride and oxide respectively in thecomposition and each being greater than zero and wherein the sum of .rand y lies between 0.1% and 15% of the weight of MS. 4. A crystallinephosphoras'claimed in claim 3 wherein the value of y is greater thanzero and is not greater than 5%.

5. A crystalline luminescent phosphor represented by the general formulaZnS(ZI1F2) :c(Zl'lO) 11 wherein m and y represent the weight of zincfluoride and zinc oxide respectively in the combined composition andwherein the sum of at and 3 lies between 0.1% to 15% of the weight ofzinc sulphide content. i

6. The method of manufacturing a luminescent phosphor compositioncomprising the steps of adding a fluoride to a sulphur compound of ametal selected from the group of metals consisting of zinc and cadmiumto form a mixture thereof, adding tosaid mixture a flux to aidcrystallization, and crystallizing said mixture by firing at atemperature between 750 C. and 1600 C. 7. The method of manufacturing aluminescent phosphor composition comprising the steps of adding afluoride of a metal selected from the group of metals consisting of zincand cadmium to a sulphide of said metal to form a mixture of saidfluoride and sulphide, adding to said mixture a flux to aidcrystallization, and crystallizing said mixture as a combined fluorideand sulphide by firing said mixture and said flux at a temperaturebetween 750 C. and 1600 C.

8. The method of manufacturing a sulphide phosphor comprising preparinga solution of a sulphide of a metal in the group consisting of zinc andcadmium having a pH ratio between 4 and 7, the said solution having amaximum impurity by weight with respect to the solution of:

precipitating a sulphide and a fluoride of said metal fromsaid solution,adding a flux to the precipitate, drying said precipitate and flux, andfiring the precipitate and flux at a temperature sufficient tocrystallize the dried precipitate as a mixed sulphide-fluoride phosphor.

9. The method of synthesizing a luminescent sulphide compound of a metalselected from the group consisting of zinc and cadmium comprising mixingwith a compound of said metal a quantity of a fluoride of an elementselected from the group consisting of said metal and hydrogen and firingsaid mixture to crystallize said compound in the presence of saidfluoride.

HUMBOLDT W. LEVERENZ.

